The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to correct gradient field distortion where an object moves with respect to the gradient non-linearities.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In MR imaging, magnetic field gradients are used to spatially encode objects. A magnetic field gradient is a linear variation along any of the principal directions of the Bz field. Non-linearities of the magnetic field gradient cause geometric distortion or “warping” of the resulting image.
It is highly desirable to extend the available imaging field-of-view (FOV) images by continuous or stepped table motion. These techniques attempt to image in the region where the gradients are mostly linear to minimize errors caused by gradient non-linearities. Such errors result in ghosting and/or blurring of the resulting images. The principal goal of acquiring images while the table is moving is to extend the usable imaging FOV beyond that which is normally achievable. However, to date the issue of correcting for gradient distortion in the presence of continuous or stepped table motion has not been adequately resolved. Previous approaches have focused on imaging over a relatively narrow region of the gradient coil where the gradients are substantially linear, thereby reducing the need for correction. However, by increasing the imaging volume to include regions of gradient non-linearity, the acquisition time for these types of scans can be greatly reduced.
In moving table imaging, the subject passes through different physical locations in the magnet during acquisition. Therefore, the subject experiences different amounts of gradient non-linearity as the subject moves from iso-center to the periphery of the gradient field. Thus, the subject is encoded with different positional errors during movement through the magnetic field. These errors can cause blurring and ghosting in the resulting images in addition to geometric distortion. That is, if the table is moving continuously during data acquisition, then each point in k-space is acquired at a different location in the sample being image. This means that each point in the subject experiences different gradient fields over the course of the data acquisition and a correspondingly different amount of distortion.
For the special case of frequency encoding along the direction of motion each phase-encoding step is acquired at a different table position corresponding to a different location in the object being imaged. In this technique, the data is first Fourier transformed along the frequency-encoding direction resulting in hybrid data. Each phase-encoding in this hybrid data can then be registered by calculating the pixel offset from the pulse sequence repetition time (TR) and the table velocity (v) and applying the appropriate shift. Further Fourier transform(s), the number of which is based on whether a 2D or 3D image is being reconstructed, can then be performed on the entire hybrid data set after the appropriate shifts have been applied to each of the phase-encodings. While this technique has proven to provide adequate images in many applications, it could be improved by opening up the FOV to include regions of increased gradient non-linearity and/or could benefit from higher quality images if a gradient non-linearity correction were employed.
It would therefore be desirable to have a method and apparatus to compensate for gradient non-linearity where the gradients vary. A specific implementation of which is moving table imaging.